[RFC PATCH v15 2/7] locking/mutex: Rework task_struct::blocked_on

John Stultz
2025-03-12T22:12:07Z

From: Peter Zijlstra <peterz@infradead.org>

Track the blocked-on relation for mutexes, to allow following this
relation at schedule time.

task
| blocked-on
v
mutex
| owner
v
task

This all will be used for tracking blocked-task/mutex chains
with the prox-execution patch in a similar fashion to how
priority inheritance is done with rt_mutexes.

For serialization, blocked-on is only set by the task itself
(current). And both when setting or clearing (potentially by
others), is done while holding the mutex::wait_lock.

Cc: Joel Fernandes <joelagnelf@nvidia.com>
Cc: Qais Yousef <qyousef@layalina.io>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Zimuzo Ezeozue <zezeozue@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Will Deacon <will@kernel.org>
Cc: Waiman Long <longman@redhat.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Metin Kaya <Metin.Kaya@arm.com>
Cc: Xuewen Yan <xuewen.yan94@gmail.com>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Suleiman Souhlal <suleiman@google.com>
Cc: kernel-team@android.com
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
[minor changes while rebasing]
Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Connor O'Brien <connoro@google.com>
[jstultz: Fix blocked_on tracking in __mutex_lock_common in error paths]
Signed-off-by: John Stultz <jstultz@google.com>
---
v2:
* Fixed blocked_on tracking in error paths that was causing crashes
v4:
* Ensure we clear blocked_on when waking ww_mutexes to die or wound.
This is critical so we don't get circular blocked_on relationships
that can't be resolved.
v5:
* Fix potential bug where the skip_wait path might clear blocked_on
when that path never set it
* Slight tweaks to where we set blocked_on to make it consistent,
along with extra WARN_ON correctness checking
* Minor comment changes
v7:
* Minor commit message change suggested by Metin Kaya
* Fix WARN_ON conditionals in unlock path (as blocked_on might already
be cleared), found while looking at issue Metin Kaya raised.
* Minor tweaks to be consistent in what we do under the
blocked_on lock, also tweaked variable name to avoid confusion
with label, and comment typos, as suggested by Metin Kaya
* Minor tweak for CONFIG_SCHED_PROXY_EXEC name change
* Moved unused block of code to later in the series, as suggested
by Metin Kaya
* Switch to a tri-state to be able to distinguish from waking and
runnable so we can later safely do return migration from ttwu
* Folded together with related blocked_on changes
v8:
* Fix issue leaving task BO_BLOCKED when calling into optimistic
spinning path.
* Include helper to better handle BO_BLOCKED->BO_WAKING transitions
v9:
* Typo fixup pointed out by Metin
* Cleanup BO_WAKING->BO_RUNNABLE transitions for the !proxy case
* Many cleanups and simplifications suggested by Metin
v11:
* Whitespace fixup pointed out by Metin
v13:
* Refactor set_blocked_on helpers clean things up a bit
v14:
* Small build fixup with PREEMPT_RT
v15:
* Improve consistency of names for functions that assume blocked_lock
is held, as suggested by Peter
* Use guard instead of separate spinlock/unlock calls, also suggested
by Peter
* Drop blocked_on_state tri-state for now, as its not needed until
later in the series, when we get to proxy-migration and return-
migration.
---
include/linux/sched.h | 5 +----
kernel/fork.c | 3 +--
kernel/locking/mutex-debug.c | 9 +++++----
kernel/locking/mutex.c | 19 +++++++++++++++++++
kernel/locking/ww_mutex.h | 18 ++++++++++++++++--
5 files changed, 42 insertions(+), 12 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 1462f2c70aefc..03775c44b7073 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1212,10 +1212,7 @@ struct task_struct {
struct rt_mutex_waiter *pi_blocked_on;
#endif

-#ifdef CONFIG_DEBUG_MUTEXES
- /* Mutex deadlock detection: */
- struct mutex_waiter *blocked_on;
-#endif
+ struct mutex *blocked_on; /* lock we're blocked on */

#ifdef CONFIG_DEBUG_ATOMIC_SLEEP
int non_block_count;
diff --git a/kernel/fork.c b/kernel/fork.c
index 735405a9c5f32..38f055082d07a 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2357,9 +2357,8 @@ __latent_entropy struct task_struct *copy_process(
lockdep_init_task(p);
#endif

-#ifdef CONFIG_DEBUG_MUTEXES
p->blocked_on = NULL; /* not blocked yet */
-#endif
+
#ifdef CONFIG_BCACHE
p->sequential_io = 0;
p->sequential_io_avg = 0;
diff --git a/kernel/locking/mutex-debug.c b/kernel/locking/mutex-debug.c
index 6e6f6071cfa27..758b7a6792b0c 100644
--- a/kernel/locking/mutex-debug.c
+++ b/kernel/locking/mutex-debug.c
@@ -53,17 +53,18 @@ void debug_mutex_add_waiter(struct mutex *lock, struct mutex_waiter *waiter,
{
lockdep_assert_held(&lock->wait_lock);

- /* Mark the current thread as blocked on the lock: */
- task->blocked_on = waiter;
+ /* Current thread can't be already blocked (since it's executing!) */
+ DEBUG_LOCKS_WARN_ON(task->blocked_on);
}

void debug_mutex_remove_waiter(struct mutex *lock, struct mutex_waiter *waiter,
struct task_struct *task)
{
+ struct mutex *blocked_on = READ_ONCE(task->blocked_on);
+
DEBUG_LOCKS_WARN_ON(list_empty(&waiter->list));
DEBUG_LOCKS_WARN_ON(waiter->task != task);
- DEBUG_LOCKS_WARN_ON(task->blocked_on != waiter);
- task->blocked_on = NULL;
+ DEBUG_LOCKS_WARN_ON(blocked_on && blocked_on != lock);

INIT_LIST_HEAD(&waiter->list);
waiter->task = NULL;
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index b36f23de48f1b..37d1966970617 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -627,6 +627,8 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
goto err_early_kill;
}

+ WARN_ON(current->blocked_on);
+ current->blocked_on = lock;
set_current_state(state);
trace_contention_begin(lock, LCB_F_MUTEX);
for (;;) {
@@ -663,6 +665,12 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas

first = __mutex_waiter_is_first(lock, &waiter);

+ /*
+ * As we likely have been woken up by task
+ * that has cleared our blocked_on state, re-set
+ * it to the lock we are trying to aquire.
+ */
+ current->blocked_on = lock;
set_current_state(state);
/*
* Here we order against unlock; we must either see it change
@@ -683,6 +691,7 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
}
raw_spin_lock_irqsave(&lock->wait_lock, flags);
acquired:
+ current->blocked_on = NULL;
__set_current_state(TASK_RUNNING);

if (ww_ctx) {
@@ -712,9 +721,11 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
return 0;

err:
+ current->blocked_on = NULL;
__set_current_state(TASK_RUNNING);
__mutex_remove_waiter(lock, &waiter);
err_early_kill:
+ WARN_ON(current->blocked_on);
trace_contention_end(lock, ret);
raw_spin_unlock_irqrestore_wake(&lock->wait_lock, flags, &wake_q);
debug_mutex_free_waiter(&waiter);
@@ -924,6 +935,14 @@ static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigne
next = waiter->task;

debug_mutex_wake_waiter(lock, waiter);
+ /*
+ * Unlock wakeups can be happening in parallel
+ * (when optimistic spinners steal and release
+ * the lock), so blocked_on may already be
+ * cleared here.
+ */
+ WARN_ON(next->blocked_on && next->blocked_on != lock);
+ next->blocked_on = NULL;
wake_q_add(&wake_q, next);
}

diff --git a/kernel/locking/ww_mutex.h b/kernel/locking/ww_mutex.h
index 37f025a096c9d..00db40946328e 100644
--- a/kernel/locking/ww_mutex.h
+++ b/kernel/locking/ww_mutex.h
@@ -284,6 +284,14 @@ __ww_mutex_die(struct MUTEX *lock, struct MUTEX_WAITER *waiter,
#ifndef WW_RT
debug_mutex_wake_waiter(lock, waiter);
#endif
+ /*
+ * When waking up the task to die, be sure to clear the
+ * blocked_on pointer. Otherwise we can see circular
+ * blocked_on relationships that can't resolve.
+ */
+ WARN_ON(waiter->task->blocked_on &&
+ waiter->task->blocked_on != lock);
+ waiter->task->blocked_on = NULL;
wake_q_add(wake_q, waiter->task);
}

@@ -331,9 +339,15 @@ static bool __ww_mutex_wound(struct MUTEX *lock,
* it's wounded in __ww_mutex_check_kill() or has a
* wakeup pending to re-read the wounded state.
*/
- if (owner != current)
+ if (owner != current) {
+ /*
+ * When waking up the task to wound, be sure to clear the
+ * blocked_on pointer. Otherwise we can see circular
+ * blocked_on relationships that can't resolve.
+ */
+ owner->blocked_on = NULL;
wake_q_add(wake_q, owner);
-
+ }
return true;
}

--
2.49.0.rc0.332.g42c0ae87b1-goog